1. Field of the Invention
The invention relates to a method for the correction of certain types of distortion in a projection lens of a microlithographic projection exposure apparatus.
2. Description of the Related Art
In the course of the production of large-scale integrated circuits and other microstructured components, several structured layers are applied onto a suitable substrate which may be, for example, a silicon wafer. With a view to structuring the layers, the latter are firstly covered with a photoresist that is sensitive to light of a particular wavelength range, for example light in the deep ultraviolet (DUV) or extreme ultraviolet (EUV) spectral range. The wafer that has been coated in this way is subsequently exposed in a projection exposure apparatus. In the course of this exposure, a pattern of structures which is located on a reticle is imaged onto the photoresist with the aid of a projection lens. Since the linear magnification in this case is generally less than 1, projection lenses of such a type are frequently also designated as reduction lenses.
After the photoresist has been developed, the wafer is subjected to an etching process or a deposition process, as a result of which the uppermost layer is structured so as to correspond to the pattern on the mask. The photoresist that has been left behind is then removed from the remaining parts of the layer. This process is repeated until all the layers on the wafer have been applied.
One of the main aims in the development of microlithographic projection exposure apparatuses consists in being able to generate structures on the wafer having increasingly smaller dimensions, in order in this way to increase the integration density of the components to be produced. One possible way to achieve this aim consists in improving the imaging quality of the projection lens through the correction of imaging errors.
The causes of imaging errors in projection lenses are varied. Imaging errors that go back to material defects or manufacturing errors are frequently particularly difficult to correct. The same holds for imaging errors that are caused by changes in the optical elements contained in the projection lens and that arise only during operation. In this connection it may be a question, for example, of temporary changes of shape resulting from local heating by the high-energy projection light. The projection light may also interact directly with the material from which the optical elements are made and may, for example, bring about permanent changes therein in the refractive index.
An imaging error that occurs frequently in projection lenses is distortion. This term is to be understood to mean, quite generally, the effect such that structures in the object plane of an optical imaging system are not all imaged onto the image plane with the same linear magnification. Although this does not impair the sharpness of the image, it has the result that the image is no longer similar to the object in the geometrical sense.
In the mathematical description of distortion the distorted image is generally compared with a required image. For individual image points it is then ascertained to what extent the actual position in the image plane deviates from the required position. These deviations can be described, for example, by vectors that specify the direction and the magnitude of the deviation. These vectors then convey a vivid picture of the distortion.
With the aid of the distortion vectors it is possible for different types of distortion to be described in particularly simple manner. Firstly, it is possible to distinguish between linear distortion, quadratic distortion and higher-order distortion. In the case of a linear distortion, the magnitude of the distortion vectors increases linearly the further they are removed from a particular reference-point which, for example, may be situated in the middle of the image field. In the case of a quadratic distortion, the magnitude of the distortion vectors depends on the product of two position coordinates, for example x2, y2 or xy. Corresponding remarks apply in respect of higher-order distortions.
In addition, the distortion may be differentiated in accordance with its symmetry properties with respect to a plane of symmetry of the projection lens, which generally also extends through the middle of the image field and subdivides the latter into two mirror-symmetric halves. Thus the distortion vectors may, for example, be disposed symmetrically with respect to this plane of symmetry in such a way that they pass into themselves again in the event of an imagined rotation of the projection lens by 180°. A linear distortion with such a twofold symmetry is also designated as anamorphism. Anamorphism can also be interpreted visually as an imaging error in which the linear magnification is different in two mutually perpendicular directions. This results in a distortion of the image, with which a circle, for example, is imaged as an ellipse. Amongst the distortions having special symmetry properties, besides the symmetric distortions there are also antisymmetric distortions.
In the case of the quadratic distortions, it is likewise possible to distinguish between symmetric and antisymmetric distortions with respect to the plane of symmetry of the projection lens. In addition, one differentiates further between sagittal and tangential distortion, depending on the directions of the distortion vectors in a polar coordinate system that is centred relative to the optical axis.
By way of measures for the correction of imaging errors, changes in the position of individual optical elements with the aid of manipulators generally enter into consideration, for example. Manipulators of such a type, which are known as such, in particular enable optical elements to be displaced along the optical axis or even perpendicular thereto, to be rotated about the optical axis, or to be tilted perpendicular thereto. A purposeful bending of optical elements has also been proposed.
For the correction of distortion in projection lenses especially it is known to tilt individual optical elements contained therein about a tilt axis perpendicular to the optical axis or to decentre them with respect to the optical axis. The term ‘decentring’ is understood to mean a translational relocation of the optical element in question in a plane disposed perpendicular to the optical axis. A deformation of individual optical elements for the purpose of correction is also possible.
Certain types of distortion, namely an error in the linear magnification and also quadratic distortion, cannot be corrected by a change in position of the reticle in the case of doubly telecentric projection lenses. Such corrections are only possible if one or more optical elements within the projection lens is/are changed in its/their position. In order, for example, to correct a tangential distortion and a sagittal distortion independently of one another, at least two manipulators have to be present which generate the requisite changes of position. These manipulators are structurally very elaborate and for this reason contribute considerably to the costs of the projection lens, since they have to guarantee a high precision of adjustment over a relatively long traversed path. In this context it is a further disadvantage that, by virtue of the change in position of optical elements, as a rule other imaging errors are generated which entail further correction measures.